Covering a pit or making a fence around it:[1]

The negative command against causing blood to spill in one’s home applies both to roofs and for anything else that poses a danger, and that a person can stumble on and die. Thus, one who has a pit [or well[2]] in his yard [or home] is obligated to make a barrier surrounding it, or alternatively to make a cover for it, in order so one not fall in it and die r”l.

The height of the fence:[3] The barrier must be a height of 10 handbreadths.

A pit filled with water:[4] The above obligation applies whether the pit is filled with water or not.

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^[1] Admur Shemiras Guf Vinefesh Halacha 3; Michaber C.M. 427:7; Rambam Rotzeiach 11:4; Sifri Ki Seitzei; Sefer Shemiras Haguf Vihanefesh [Lerner] 218:2

[2] Michaber ibid

[3] Admur ibid; Michaber ibid; Rambam ibid; Chinuch Mitzvah 546

Other opinions: Some Poskim rule that the barrier must be a minimum height of only three Tefachim. [Semag Asei 79; Yireim 234; Orchos Chaim 16; Semak 152; Rabbeinu Yerucham 1:1-5]

[4] Admur ibid; Michaber ibid; Rambam ibid

[5] See Imreiy Yaakov 10:10; Shemiras Hanefesh Kehilchasa 1:3

[6] Minchas Chinuch Mitzvah 546; Maaseh Rav 100; Devar Avraham 1:37; Tosafos Reim 234; Shemiras Hanefesh Kehilchasa 1:3

[7] Ben Ish Chai Pinchas

[8] Chayeh Adam 15:24 and that so is the opinion of the Rambam and Shulchan Aruch, brought in Pischeiy Teshuvah C.M. 427:1; Haemek Sheila Sheilasa 145:17; Chazon Ish Likkutim 18:9; Rav Chaim Kanievsky in Miseches Mezuzah Hilchos Meakah Shaar Hatziyon 55; Aruch Hamishpat p. 28; Viasisa Miakah 3:1

[9] The reason: As this command is not included in the positive command of building a fence but rather only in the general positive and negative command to prevent danger, and hence it does not deserve a blessing. [See Chayeh Adam ibid]

[10] As Admur includes this obligation only in the general positive and negative command to prevent danger, and not in the positive command of building a fence.

[11] Pashut! Shut Baruch Omer 152; Admur ibid clearly states that a pit with water in a courtyard must be covered which is exactly what a pool is.

Other opinions: Certain Melaktim of today have written various leniencies regarding this matter, claiming that since a pool is apparent to everyone it does not need a fence. [See Sefer Veasisa Meakah 5:9 footnote 18 based on his understanding of Chazon Ish C.M. Likkutim 18 that only an inconspicuous pit requires a fence, while a large pit which is viewable by all does not. So also writes Rabbi Bressman in the end of his Sefer on Hilchos Mezuzah, actually sourcing it from the Chazon Ish. This is clearly not accurate as the Chazon Ish never discusses this subject at all, not that of a pool nor of a large pit in a public area.] In my opinion, this ruling is not accurate. Aside from the fact that the statistics speak for themselves regarding the amount of pool drownings and deaths that occur every year, which are exacerbated when there is no fence around the pool, there’s also no room in Halacha to negate its requirement. The wording of the law plainly reads that a pit with water even in a private courtyard must be fenced. It does not make any differentiation between size or a public domain versus a private domain. If their opinion were correct according to Halacha, then the Poskim would’ve made this clear and differentiated between a small versus large pit and a public versus private domain. The fact that they did not differentiate in this matter and explicitly wrote that it applies to a courtyard, is the biggest proof necessary to show that they did not hold of their logic of differentiation. Furthermore, even those who write of Halachic leniency, agree that in practicality the pool should be fenced, and that so is the widespread custom. Therefore, G-d forbid, no leniency should be learned from this and every pool must be fenced and well-guarded to prevent the needless drownings and deaths that happen every year by unguarded pools, may G-d save us.